Measuring the heart pulse rate (heartbeat) of a user is a topic of significant importance in personal health care, sports, stress monitoring, and other applications. If a heart pulse rate or heartbeat sensor can be packaged into a device, for example, a wristwatch, the sensor can be worn in a convenient position for the user. Such a device can be worn for activities ranging from intensive sports to sleeping.
Heart pulse rates provide information about the general health status of an individual and can indicate his or her general fitness, mental state, and/or level of physical activity. In addition, heart pulse rates can be used to detect cardiac arrhythmia, such as bradychardia and tachycardia for instance. Heart pulse rates also provide information about other specific medical conditions, such as the onset of an epileptic seizure, for instance.
Besides heart pulse and heart pulse rate monitoring, the heart rate variability (HRV) is also of significant interest. It is often claimed that the HRV carries information about autonomic regulation of cardiac activity. It is believed that, as a result of an arousing event, the heart rate will increase, in order to prepare the body for physical activity. In addition, it has been shown that the HRV has the potential to indicate the quality of a person's sleep. Other investigations highlight the importance of using the HRV as a parameter in calculating stress in drivers or military personnel. HRV also has been known to be affected by smoking, alcohol and caffeine consumption, as well as age and gender. In diseases, HRV is known to be affected by myocardial infarction, ventricular arrhythmias, hypertension, diabetes mellitus, and heart failure. Typically, analysis of the HRV is done using what is called a “footprint,” which is a recording of the heart rate (HR) and HRV over a period of 24 hours.
However, obtaining an accurate measurement of an active person's pulse rate at the wrist is a complex process. This is because of the presence of artefacts produced by body motion. These artefacts are detected by the heart pulse sensor as noise. In many cases, this noise can produce signals of sufficient amplitude to completely mask the heart pulse signal which is to be measured.
U.S. Pat. No. 5,807,267 (Bryars et al.) discloses a heart rate monitor that is included in a wrist band. The heart rate monitor comprises two piezoelectric sensors arranged in a side-by-side configuration and is placed so that the sensors detect a pulse in an artery over which the sensors are placed. One of the sensors is used as a primary sensor and the other as a reference or background sensor. Signals generated by the reference or background sensor are digitally subtracted from signals generated by the primary sensor. The subtraction of the two signals reduces the effects due to motion of the user whilst using the heart rate monitor.